Conventional approaches to implementing crystal or other resonant oscillators generally involve manufacturing a fixed capacitive load that is attached to the resonator. The fixed load is generally fabricated during the manufacturing process. As a result, the accuracy of the manufacturing process in a conventional oscillator plays an important role in determining the accuracy of the output clock, as does the manufacturing accuracy of the resonator.
Referring to FIG. 1, a circuit 10 illustrates one such conventional approach. The circuit 10 generally comprises a resistor 12, an inverter 14, a resistor 16, an inverter 18, a capacitor Cl, a capacitor C2 and a crystal 20. One side of the crystal 20 is coupled between the capacitor Cl and the input of the inverter 14. A second side of the oscillator 20 is coupled to the capacitor C2 and the resistor 16. The resistor 16 is also coupled between the output of the inverter 14 and the input of the inverter 18. The output of the inverter 18 presents a signal FOUT. The signal FOUT oscillates at a particular frequency that is generally a function of the size of the capacitors Cl and C2. As a result, to change the frequency of oscillation of the signal FOUT, the size of the capacitors Cl and C2 must be adjusted during fabrication of the circuit 10.
Manufacturing processes generally have a certain amount of inherent inaccuracy. Since the circuit 10 relies on the accuracy of the manufacturing process to produce the signal FOUT, the overall accuracy of the circuit 10 may not be acceptable in certain design applications having specifications that demand extremely accurate tolerances for the frequency of oscillation of the signal FOUT. A computer motherboard is one example of such an application.
Oscillators used in watches may have configuration bits configured to trim the accuracy of the oscillator after production. The oscillator may typically be tuned to the range of parts per million. However, the oscillators used in watches are typically low speed oscillators and are generally not appropriate for high speed computer applications. Furthermore, the tuning is generally achieved with a mechanical trimming capacitor which is less desirable than electronic trimming.